Construction method of excavation and unloading around existing tunnels

Abstract

The present invention discloses a construction method for excavation and unloading around an existing tunnel, comprising the following steps: S1. obtaining a deep-shallow buried boundary line or a pressure arch range line of the existing tunnel based on geological survey data and existing tunnel data and using it as a protection critical line of the existing tunnel; S2. determining the thickness and range of the rock and soil layer to be excavated and unloaded around the existing tunnel based on the site design elevation requirements; S3. comparing the positional relationship between the protection critical line of the existing tunnel and the proposed excavation and unloading range in the plane and vertical directions, and judging whether the proposed excavation and unloading range intrudes into the protection critical line of the existing tunnel; S4. adopting a corresponding excavation method based on the intrusion situation of the proposed excavation and unloading range around the existing tunnel; the present invention is mainly based on the bearing capacity of the surrounding rock around the existing tunnel, reasonably selects the corresponding surrounding rock and soil excavation and unloading step sequence, and effectively utilizes the characteristic of the tunnel surrounding rock bearing arch, so as to minimize the displacement and deformation of the tunnel and ensure the rapid progress of the rock and soil excavation construction around the tunnel.

Description

Technical Field

[0001] The present invention relates to the field of urban tunnels, and in particular to a construction method for excavating and unloading the periphery of an existing tunnel. Background Art

[0002] With the increasing expansion and construction of cities, the development and utilization of urban underground space is also increasing. With the continuous development of underground space and the vigorous construction of subway tunnels, more and more construction foundation pits are inevitably adjacent to or intersecting with existing tunnels. Excavation near existing urban tunnels will induce stratum stress release: as the rock and soil around the tunnel are excavated, the surrounding area of ​​the tunnel begins to unload, and the original stress field of the surrounding rock changes. In its unconstrained and unreinforced state, the surrounding rock will undergo stress adjustment and then reach equilibrium again. During this stress re-equilibrium process, the surrounding rock will deform, causing the tunnel located in the surrounding rock to also shift and deform.

[0003] Therefore, how to rationally unload the surrounding excavation of existing tunnels has become a hot topic in urban underground engineering. However, current research focuses on load transfer structures or foundation pit support structures for unloading the surrounding excavation of existing tunnels. There is less research on how to utilize the critical protection range of existing tunnels to determine a reasonable excavation sequence around existing tunnels. Consequently, there is a lack of construction methods that fully utilize the bearing capacity of the surrounding rock of the existing tunnel to unload the surrounding excavation. Summary of the Invention

[0004] In view of this, the present invention provides a construction method for excavation and unloading around an existing tunnel. The method is mainly based on the bearing capacity of the surrounding rock around the existing tunnel, reasonably selects the corresponding excavation and unloading sequence of the surrounding rock and soil, and effectively utilizes the bearing arch characteristic of the tunnel surrounding rock to minimize the displacement and deformation of the tunnel and ensure the rapid progress of the rock and soil excavation construction around the tunnel.

[0005] The present invention provides a construction method for excavating and unloading around an existing tunnel, comprising the following steps:

[0006] S1. Based on geological survey data and existing tunnel data, determine the deep-shallow burial boundary or pressure arch range of the existing tunnel and use it as the critical protection line of the existing tunnel;

[0007] S2. Determine the thickness and extent of the rock and soil layer to be excavated and unloaded around the existing tunnel based on the project site redline and site design elevation requirements;

[0008] S3. Compare the horizontal and vertical positions of the critical protection line of the existing tunnel and the proposed unloading excavation area to determine whether the proposed unloading excavation area intrudes into the critical protection line of the existing tunnel;

[0009] S4. Adopt appropriate excavation methods based on the intrusion situation within the planned unloading area around the existing tunnel.

[0010] Furthermore, in step S3, by comparing the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range in the horizontal and vertical directions, the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is adjusted to: A. The planned excavation unloading range around the existing tunnel does not intrude into the protection critical line of the existing tunnel; B. The planned excavation unloading range around the existing tunnel intrudes into the protection critical line of the top of the existing tunnel but does not intrude into the protection critical lines on both sides thereof; C. The planned excavation unloading range around the existing tunnel intrudes into the protection critical lines of the top and both sides of the existing tunnel; D. The planned excavation unloading range around the existing tunnel intrudes into the protection critical line of one side of the existing tunnel;

[0011] The positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range can only exist in one of the above-mentioned states at the same time.

[0012] Further, in step S4, when the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is in a state where the planned excavation unloading range around the existing tunnel does not invade the protection critical line of the existing tunnel, the excavation method within the planned excavation unloading range adopts layered and segmented excavation construction to the site design elevation.

[0013] Furthermore, when the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is such that the planned excavation unloading range around the existing tunnel intrudes into the protection critical line of the top of the existing tunnel but does not intrude into the protection critical lines on both sides thereof, or intrudes into both the top and both sides of the existing tunnel, a load transfer structure is provided within the protection critical line of the existing tunnel;

[0014] The load transfer structure includes a pile foundation and a transfer beam. The pile foundation adopts isolation measures in the part above the 45° stress diffusion line located at the bottom of the existing tunnel structure. The transfer beam and the rock mass at the bottom of the beam are hollowed out and deformation joints are set.

[0015] Furthermore, in step S4, when the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is such that the planned excavation unloading range around the existing tunnel simultaneously intrudes into the protection critical lines on the top and both sides of the existing tunnel, the excavation method within the planned excavation unloading range adopts the following steps:

[0016] 1) Construction Excavation Zone I: The area between the original ground line and the top elevation of the existing tunnel's critical protection line is designated as Excavation Zone I. Excavation Zone I is excavated in layers and sections.

[0017] 2) Construction Excavation Zone II: The rock and soil areas on both sides of the critical line of protection of the existing tunnel within the elevation range between the construction interface at the top of the load transfer structure and Excavation Zone I are designated as Excavation Zone II. Excavation Zone II is excavated symmetrically in layers and sections from far to near.

[0018] 3) Construction Excavation Zone III: The rock and soil area between the existing tunnel top protection critical line and the load transfer structure top construction interface is designated as Excavation Zone III. In Excavation Zone III, layered and segmented excavation is adopted, and the pile foundation of the load transfer structure is constructed using the skip pile method, ultimately completing the construction of the entire load transfer structure.

[0019] 4) Construction Excavation Zone IV: The area between the construction interface of the load transfer structure and the design elevation of the site is set as Excavation Zone IV. Excavation Zone IV is excavated symmetrically in layers and sections from far to near.

[0020] Furthermore, in step S4, when the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is in a state of invading the protection critical line of the top of the existing tunnel but not invading the protection critical lines on both sides thereof, the excavation method within the planned excavation unloading range adopts the following steps:

[0021] A. Construction Excavation Area I: The area between the original ground line and the site design elevation is designated as Excavation Area I. Excavation Area I is excavated in layers and sections.

[0022] B. Construction of load transfer structure: At this time, the design elevation of the site is the top construction interface of the load transfer structure. The top construction interface of the load transfer structure adopts the pile jumping method to construct the load transfer structure pile foundation, and finally complete the construction of the entire load transfer structure.

[0023] Furthermore, when the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is such that the planned excavation unloading range around the existing tunnel intrudes into the protection critical line on one side of the existing tunnel, a foundation pit support structure is set within the planned excavation unloading range.

[0024] Furthermore, in step S4, when the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is such that the planned excavation unloading range around the existing tunnel intrudes into the protection critical line on one side of the existing tunnel, the excavation method within the planned excavation unloading range adopts the following steps:

[0025] a. The site design elevation of the existing tunnel top is to be excavated in layers and sections to the existing tunnel top above the critical protection line;

[0026] b. Construct foundation pit support structures in the planned excavation and unloading area beside the existing tunnel;

[0027] c. Excavate the unloading area beside the existing tunnel in layers and sections from far to near.

[0028] Furthermore, in step b, the foundation pit support structure is arranged in the form of a setback outside the protection critical line of the existing tunnel or is directly arranged in the planned excavation and unloading range within the protection critical line of the existing tunnel.

[0029] Furthermore, a number of flow operation areas are provided along the longitudinal direction of the existing track within the planned excavation and unloading range, and the flow operation areas are used to reduce the uneven longitudinal force of the interval tunnel.

[0030] Beneficial effects of the present invention: The present invention discloses a construction method for excavation and unloading around an existing tunnel, which is mainly based on the bearing capacity of the surrounding rock around the existing tunnel, reasonably selects the corresponding excavation and unloading sequence of the surrounding rock and soil, and effectively utilizes the bearing arch characteristic of the tunnel surrounding rock to minimize the displacement and deformation of the tunnel and ensure the rapid excavation construction of the rock and soil around the tunnel. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0032] Figure 1 This is a typical excavation cross section (I) of the present invention;

[0033] Figure 2 This is a typical excavation cross section (II) of the present invention;

[0034] Figure 3 This is a typical excavation cross section (three) of the present invention;

[0035] Figure 4 This is a typical excavation cross section (four) of the present invention;

[0036] The accompanying drawings are marked as follows: 1-original ground line; 2-elevation position of the top of the protection critical line of the existing tunnel; 3-construction interface at the top of the load transfer structure; 4-site design elevation position; 5-protection critical line of the existing tunnel; 6-load transfer structure; 7-existing tunnel; 8-site design elevation of the top of the existing tunnel; 9-foundation pit support structure; 10-excavation area I; 11-excavation area II; 12-excavation area III; 13-excavation area IV. DETAILED DESCRIPTION

[0037] As shown in the figure, the present invention provides a construction method for excavating and unloading around an existing tunnel, comprising the following steps:

[0038] S1. Based on geological survey data and existing tunnel data, the existing tunnel 7 is obtained as the deep and shallow buried boundary line or pressure arch range line and serves as the protection critical line 5 of the existing tunnel;

[0039] S2. Determine the thickness and extent of the rock and soil layer to be excavated and unloaded around the existing tunnel based on the project site redline and site design elevation requirements;

[0040] S3. Compare the positional relationship between the protection critical line 5 of the existing tunnel and the proposed excavation unloading range in the horizontal and vertical directions to determine whether the proposed excavation unloading range intrudes into the protection critical line of the existing tunnel;

[0041] S4. Adopt appropriate excavation methods based on the intrusion situation within the planned unloading area around the existing tunnel.

[0042] In this embodiment, the deep-shallow buried boundary line of the existing tunnel in step S1 is mainly obtained by the tunnel surrounding rock pressure calculation method in the "Highway Tunnel Design Code" or the "Railway Tunnel Design Code", and the pressure arch range line of the existing tunnel is a gradient trend line formed by the horizontal stress in the rock and soil unit around the existing tunnel being greater than the vertical stress obtained by numerical simulation software.

[0043] In this embodiment, in step S3, by comparing the positional relationship between the protection critical line 5 of the existing tunnel and the planned excavation unloading range in the horizontal and vertical directions, the positional relationship between the protection critical line 5 of the existing tunnel and the planned excavation unloading range is made to be in the following states: A. The planned excavation unloading range around the existing tunnel 7 does not invade the protection critical line of the existing tunnel; B. The planned excavation unloading range around the existing tunnel 7 invades the protection critical line 5 of the top of the existing tunnel but does not invade the protection critical lines on both sides thereof; C. The planned excavation unloading range around the existing tunnel 7 invades the protection critical lines of the top and both sides of the existing tunnel at the same time; D. The planned excavation unloading range around the existing tunnel 7 invades the protection critical line of one side of the existing tunnel. The above four states are possible state categories of the protection critical line 5 of the existing tunnel and the planned excavation unloading range in the horizontal and vertical directions, all of which are obtained by comparing the positional relationship between the protection critical line 5 of the existing tunnel and the planned excavation unloading range. At the same time, the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range will only have one of the above four states.

[0044] In this embodiment, in step S4, when the positional relationship between the protection critical line 5 of the existing tunnel and the proposed excavation unloading range is in a state where the proposed excavation unloading range around the existing tunnel does not invade the protection critical line 5 of the existing tunnel, the proposed excavation unloading range around the existing tunnel 7 and the protection critical line 5 of the existing tunnel do not interfere with each other. Therefore, in this state, the excavation method within the proposed excavation unloading range can adopt the layered and segmented method in conventional technology, which will not be elaborated here, until the layered and segmented excavation construction reaches the site design elevation 4 required by the project.

[0045] In this embodiment, when the positional relationship between the protection critical line 5 of the existing tunnel and the planned excavation unloading range is in a state where the planned excavation unloading range around the existing tunnel 7 invades the protection critical line of the top of the existing tunnel but does not invade the protection critical lines on both sides thereof, or invades the top and both sides of the existing tunnel at the same time, a load conversion structure 6 is provided in the protection critical line 5 of the existing tunnel; the load conversion structure 9 includes a pile foundation and a conversion beam, and the pile foundation adopts isolation measures in the part above the 45° stress diffusion line located at the bottom of the existing tunnel 7 structure, and the rock mass at the bottom of the conversion beam and the beam is hollowed out and a deformation joint is provided.

[0046] Among them, the load transfer structure 6 is a pile-beam structure, which is mainly used to prevent the original stress field of the surrounding rock around the existing tunnel 7 from changing, thereby affecting the displacement and deformation of the existing tunnel 7 along with the surrounding rock; and the load transfer structure 9 is mainly composed of a pile foundation and a transfer beam. The pile foundation adopts isolation measures in the part above the 45° stress diffusion line at the bottom of the existing tunnel 7 structure, which is a conventional technical means, and the isolation measures generally adopt felt isolation, which is mainly used to prevent the pile foundation from transferring load to the surrounding rock mass; by hollowing out the transfer beam and the rock mass at the bottom of the beam and setting deformation joints, the site load above the load transfer structure 6 can be effectively prevented from being transferred to the stress field of the rock and soil layer around the existing tunnel 7 inside the load transfer structure 6, thereby preventing the stress field of the rock and soil layer around the existing tunnel 7 inside the load transfer structure 6 from being damaged; by setting up the load transfer structure 6, it can be effectively prevented that the construction of the planned excavation and unloading range within the protection critical line 5 of the existing tunnel will not have too much impact on the application field of the surrounding rock around the existing tunnel, thereby avoiding affecting the displacement and deformation of the existing tunnel 7 along with the surrounding rock around the existing tunnel.

[0047] In this embodiment, in step S4, combined with Figure 1 As shown, when the positional relationship between the protection critical line 5 of the existing tunnel and the planned excavation unloading range is such that the planned excavation unloading range around the existing tunnel simultaneously intrudes into the protection critical lines on the top and both sides of the existing tunnel, the excavation method within the planned excavation unloading range adopts the following steps:

[0048] 1) Construction excavation area I10: The area between the original ground line 1 and the top elevation position 2 of the existing tunnel's critical protection line is designated as excavation area I10. Excavation area I10 is excavated in layers and sections from top to bottom.

[0049] 2) Construction Excavation Area II11: The rock and soil areas on both sides of the existing tunnel protection critical line 5 within the elevation range between the construction interface 3 at the top of the load transfer structure and the excavation area I10 are designated as Excavation Area II11. Excavation Area II11 is excavated symmetrically in layers and sections from far to near. Excavation from far to near means excavating from the position farthest from the existing tunnel protection critical line 5 to the position close to the existing tunnel protection critical line 5 within the planned excavation unloading range.

[0050] 3) Construction Excavation Area III12: The rock and soil area between the top elevation position 2 of the existing tunnel's critical protection line and the construction interface 3 at the top of the load transfer structure is designated as Excavation Area III12. Layered and segmented excavation is used in Excavation Area III12, and the pile foundation of the load transfer structure is constructed using the skip pile method. Finally, the construction of the entire load transfer structure 6 is completed.

[0051] 4) Construction excavation area IV13: The area between the load transfer structure construction interface 3 and the site design elevation position 4 is set as excavation area IV13. Excavation area IV13 adopts symmetrical excavation in layers and sections from far to near.

[0052] In this embodiment, in step S4, when the positional relationship between the protection critical line 5 of the existing tunnel and the planned excavation unloading range is in a state of invading the protection critical line of the top of the existing tunnel but not invading the protection critical lines on both sides thereof, the excavation method within the planned excavation unloading range adopts the following steps:

[0053] A. Construction Excavation Area I 11: The area between the original ground line 1 and the site design elevation position 4 is set as Excavation Area I. Excavation Area I adopts layered and segmented excavation;

[0054] B. Construction of load transfer structure: At this time, the site design elevation position 4 is the top construction interface 3 of the load transfer structure. The top construction interface 3 of the load transfer structure adopts the pile jumping method to construct the load transfer structure pile foundation, and finally completes the construction of the entire load transfer structure.

[0055] In this embodiment, when the positional relationship between the protection critical line 5 of the existing tunnel and the planned excavation unloading range is in a state where the planned excavation unloading range around the existing tunnel intrudes into the protection critical line on one side of the existing tunnel, a foundation pit support structure 9 is set within the planned excavation unloading range; since the planned excavation unloading range around the existing tunnel only intrudes into the protection critical line on one side of the existing tunnel, it is necessary to set a corresponding foundation pit support structure 9 on this side to prevent the excavation position from affecting the stress field of the rock and soil layer on this side of the existing tunnel.

[0056] In this embodiment, in step S4, when the positional relationship between the protection critical line 5 of the existing tunnel and the planned excavation unloading range is such that the planned excavation unloading range around the existing tunnel 7 intrudes into the protection critical line on one side of the existing tunnel 7, the excavation method within the planned excavation unloading range adopts the following steps:

[0057] a. Excavate in layers and sections to the site design elevation 8 of the existing tunnel top above the existing tunnel protection critical line 5;

[0058] b. Construct foundation pit support structure 9 in the planned excavation and unloading area beside the existing tunnel;

[0059] c. Excavate the unloading area beside the existing tunnel in layers and sections from far to near.

[0060] In this embodiment, in step b, the foundation pit support structure 9 is set in the form of a setback outside the protection critical line 5 of the existing tunnel or directly set in the planned excavation unloading range within the protection critical line 5 of the existing tunnel; Figure 2 and Figure 3As shown in the figure, when the positional relationship between the protection critical line 5 of the existing tunnel and the planned excavation unloading range is in a state where the planned excavation unloading range around the existing tunnel intrudes into the protection critical line on one side of the existing tunnel, it is necessary to construct a foundation pit support structure 9 on this side. The conventional foundation pit support structure 9 adopts Figure 3 The foundation pit support structure 9 is constructed; if the construction conditions permit, Figure 2 The foundation pit support structure 9 has a better protection effect on the tunnel structure, among which Figure 2 The foundation pit support structure 9 is arranged in the form of a setback (divided into steps), which can prevent the planned excavation and unloading range around the existing tunnel from invading the existing tunnel protection critical line.

[0061] In this embodiment, several flow operation areas are set up along the longitudinal direction of the existing track within the planned excavation and unloading range, and the flow operation areas are used to reduce the uneven longitudinal force of the interval tunnel; when the construction of the rock and soil above the protection critical line of the existing tunnel is completed, the flow operation is carried out in sections according to the flow operation areas, which can ensure that the excavation steps of each front and rear flow area in the longitudinal direction do not exceed a certain height, thereby effectively reducing the uneven longitudinal force of the interval tunnel.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A construction method for excavating and unloading around an existing tunnel, characterized by: The steps include: S1. Based on geological survey data and existing tunnel data, determine the deep-shallow burial boundary or pressure arch range of the existing tunnel and use it as the critical protection line of the existing tunnel; S2. Determine the thickness and extent of the rock and soil layer to be excavated and unloaded around the existing tunnel based on the project site redline and site design elevation requirements; S3. Compare the horizontal and vertical positions of the existing tunnel's critical protection line and the proposed unloading excavation area to determine whether the proposed unloading excavation area intrudes into the existing tunnel's critical protection line. Specifically: By comparing the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range in the horizontal and vertical directions, the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is set to: A. The planned excavation unloading range around the existing tunnel does not intrude into the protection critical line of the existing tunnel; B. The planned excavation unloading range around the existing tunnel intrude into the protection critical line of the top of the existing tunnel but does not intrude into the protection critical lines on both sides thereof; C. The planned excavation unloading range around the existing tunnel intrude into the protection critical lines of the top and both sides of the existing tunnel; D. The planned excavation unloading range around the existing tunnel intrude into the protection critical line of one side of the existing tunnel. The positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range can only exist in one of the above states at the same time. S4. Based on the intrusion situation of the proposed unloading area around the existing tunnel, adopt the corresponding excavation method, specifically: When the positional relationship between the protection critical line of the existing tunnel and the proposed excavation unloading range is such that the proposed excavation unloading range around the existing tunnel does not intrude into the protection critical line of the existing tunnel, the excavation method within the proposed excavation unloading range is to adopt layered and segmented excavation to the site design elevation; When the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is such that the planned excavation unloading range around the existing tunnel simultaneously intrudes into the protection critical lines on the top and both sides of the existing tunnel, the excavation method within the planned excavation unloading range adopts the following steps: 1) Construction Excavation Zone I: The area between the original ground line and the top elevation of the existing tunnel's critical protection line is designated as Excavation Zone I. Excavation Zone I is carried out in layers and sections. 2) Construction Excavation Zone II: The rock and soil areas on both sides of the critical protection line of the existing tunnel within the elevation range between the construction interface at the top of the load transfer structure and Excavation Zone I are designated as Excavation Zone II. Excavation Zone II is excavated symmetrically in layers and sections from far to near. 3) Construction Excavation Area III: The rock and soil area between the existing tunnel top protection critical line and the load transfer structure top construction interface is designated as Excavation Area III. In Excavation Area III, layered and segmented excavation is adopted, and the pile foundation of the load transfer structure is constructed using the skip pile method, ultimately completing the construction of the entire load transfer structure. 4) Construction Excavation Zone IV: The area between the construction interface of the load transfer structure and the design elevation of the site is designated as Excavation Zone IV. Excavation Zone IV is excavated symmetrically in layers and sections from far to near. When the positional relationship between the protection critical line of the existing tunnel and the unloading range to be excavated is such that the protection critical line of the top of the existing tunnel is intruded but the protection critical lines on both sides thereof are not intruded, the excavation method within the unloading range to be excavated adopts the following steps: A. Construction Excavation Area I: The area between the original ground line and the site design elevation is designated as Excavation Area I. Excavation Area I is excavated in layers and sections. B. Construction of load transfer structure: At this point, the site design elevation is the top construction interface of the load transfer structure. The pile foundation of the load transfer structure is constructed using the skip pile method at the top construction interface of the load transfer structure, ultimately completing the construction of the entire load transfer structure. When the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is such that the planned excavation unloading range around the existing tunnel intrudes into the protection critical line on one side of the existing tunnel, the excavation method within the planned excavation unloading range adopts the following steps: a. The site design elevation of the existing tunnel top is to be set above the critical protection line of the existing tunnel by layered and segmented excavation; b. Construct foundation pit support structures in the planned excavation and unloading area beside the existing tunnel; c. Excavate the unloading area beside the existing tunnel in layers and sections from far to near.

2. The construction method for excavating and unloading around an existing tunnel according to claim 1, characterized in that: When the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is such that the planned excavation unloading range around the existing tunnel intrudes into the protection critical line of the top of the existing tunnel but does not intrude into the protection critical lines on both sides thereof, or simultaneously intrudes into the protection critical lines on both sides of the existing tunnel, a load transfer structure is provided within the protection critical line of the existing tunnel; The load transfer structure includes a pile foundation and a transfer beam. The pile foundation adopts isolation measures in the part above the 45° stress diffusion line located at the bottom of the existing tunnel structure. The transfer beam and the rock mass at the bottom of the beam are hollowed out and deformation joints are set.

3. The construction method for excavating and unloading around an existing tunnel according to claim 1, characterized in that: When the positional relationship between the protection critical line of the existing tunnel and the planned excavation unloading range is in a state where the planned excavation unloading range around the existing tunnel intrudes into the protection critical line on one side of the existing tunnel, a foundation pit support structure is set within the planned excavation unloading range.

4. The construction method for excavating and unloading around an existing tunnel according to claim 3, characterized in that: In step b, the foundation pit support structure is set in the form of a setback outside the protection critical line of the existing tunnel or is directly set in the planned excavation and unloading range within the protection critical line of the existing tunnel.

5. The construction method for excavating and unloading around an existing tunnel according to claim 1, characterized in that: Several flow operation areas are arranged along the longitudinal direction of the existing track within the planned excavation and unloading range, and the flow operation areas are used to reduce the uneven longitudinal force of the section tunnel.

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